Ion mobility spectrometer

ABSTRACT

A spray area in which a large number of droplets of a liquid sample sprayed from a spray nozzle (3) is separated from the tip of a needle electrode (14) for corona discharge by a sufficiently large distance, with a grid electrode (15) facing the needle electrode (14) placed in between. Ring electrodes (16) for creating an electric field which drives primary ions that should react with the sample and generate sample-derived ions are provided within an ion chamber (10) between the grid electrode (15) and the spray area. Primary ions generated by corona discharge within the space between the needle electrode (14) and the grid electrode (15) pass through the opening of the grid electrode (15), reach the spray area under the effect of the electric field, and ionize sample components. Since the droplets are prevented from adhering to the needle electrode (14), the corona discharge is maintained in a stable state. The resultant primary ions are efficiently transported and used for the ionization of the sample. Therefore, no spike noise due to an unstable corona discharge occurs, so that a high-quality spectrum can be obtained.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 15/527,192, filed May 16, 2017, which is a National Stage ofInternational Application No. PCT/JP2014/080318 filed Nov. 17, 2014, thecontents of all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to an ion mobility spectrometer, and morespecifically, to an ion mobility spectrometer provided with anatmospheric pressure ion source, for producing ions from a compound in aliquid sample in an ambience of atmospheric pressure and for analyzingthe ions using an ion mobility.

BACKGROUND ART

When an ion produced from a compound in a sample is made to move in agas medium (or liquid medium) by the effect of an electric field, theion moves at a speed proportional to the mobility which is determined bythe strength of the electric field, size of the ion and other factors.Ion mobility spectrometry is a measurement method utilizing this ionmobility for an analysis of a compound. Analyzing devices employing thismeasurement method are generally called the “ion mobility spectrometer”,“ion mobility meter” or otherwise. In the following description, an ionmobility spectrometer is referred to as an “IMS device”.

Commonly used IMS devices include an ion source for ionizing compoundmolecules in a sample, a drift region formed within a housing having acylindrical form (or other appropriate forms) for separating ionsaccording to their ion mobility, and a detector for detecting the ionswhich have travelled through the drift region (for example, see PatentLiterature 1). Normally, a uniform electric field which exhibits adownward potential gradient in the direction in which the ion travels(ion-moving direction), i.e. which has the effect of accelerating theions, is formed within the drift region. Additionally, a stream ofneutral gas (which is normally an inert gas) is formed in the oppositedirection to the accelerating direction by the electric field, i.e. theion-moving direction.

The ions produced in the ion source and introduced into the drift regiontravel along the downward potential gradient while colliding with theneutral gas flowing in the opposite direction. During this movement, theions are temporally separated according to their ion mobility whichdepends on the size, three-dimensional structure, electric charge andother properties of the ions. Ions having different ion mobilities reachthe detector having certain intervals of time. If the electric fieldwithin the drift region is uniform, it is possible to calculate thecollision cross-section between an ion and the neutral gas based on thedrift time required for the ion to pass through the drift region.

In the case of analyzing a compound in a gas sample using an IMS device,an ion source which ionizes the compound using beta rays emitted from aradioactive isotope, such as ⁶³Ni, or an atmospheric pressure ion sourcewhich uses corona discharge, or other types of ion sources are commonlyused (see Patent Literatures 1 and 2). Such an IMS device can be used asa detector for a gas chromatograph (GC). A GC-IMS in which an IMS deviceis connected to the exit port of the column of a GC has been practicallyused. However, the range of substances that can be detected with GC-IMSis limited to volatile substances that can be vaporized in the sampleinjection section of the GC. Accordingly, in order to enable thedetection of a wider range of substances inclusive of hard-to-volatileand non-volatile substances, an LC-IMS which uses an IMS device as thedetector for a liquid chromatograph (LC) has been developed.

In the LC-IMS, it is necessary to produce gas-phase ions from a compoundin a liquid sample in the ion source of the IMS device. For thispurpose, an ion source which employs atmospheric pressure ionization isused, such as the atmospheric pressure chemical ionization (APCI),electrospray ionization (ESI) or atmospheric pressure photoionization(APPI), all of which are also commonly used in liquid chromatograph massspectrometers (LS-MS).

CITATION LIST Patent Literature

Patent Literature 1: JP 2005-174619 A

Patent Literature 2: JP 2009-2815 A

SUMMARY OF INVENTION Technical Problem

In any of these atmospheric pressure ion sources, a liquid sample whichcontains a compound to be analyzed is sprayed into an ambience ofatmospheric pressure so as to produce gaseous ions originating from thetarget compound while vaporizing the sample solvent, i.e. whilepromoting the desolvation. However, depending on the flow rate and/orsolvent composition of the liquid sample, it may be difficult for thesolvent to vaporize from the droplets formed by the spraying process.For example, organic solvents (e.g. acetonitrile) and water are commonlyused as the mobile phase for an LC. Water has a higher boiling pointthan organic solvents. Therefore, if the mobile phase has a considerablyhigh percentage of water, the desolvation does not progress efficiently.

In the LC-IMS, there is normally no pressure difference between the ionsource and the drift region; actually, the neutral gas supplied in theopposite direction to the ion-moving direction within the drift regiongently overflows from the drift region toward the ion source. Therefore,the droplets formed by the spraying process in the ion source tend tostay within the ion source.

For example, in the APCI ion source, when it is difficult for thesolvent in the droplets to vaporize, the droplets formed by the sprayingprocess become more likely to adhere to the needle electrode for coronadischarge. The adhered droplets decrease the strength of the electricfield around the tip of the needle electrode, making it difficult tomaintain the corona discharge in a stable state. As a result, theionization of the compound to be analyzed also becomes unstable, whichcauses a fluctuation in the intensity of the detected signal or ageneration of spike noises, making the eventually obtained spectrum (ionmobility spectrum) or chromatogram less reliable. FIG. 9 is an exampleof the chromatogram observed with a conventional LC-APCI-IMS device inwhich spike noises are present.

Additionally, an LC-IMS device employing an atmospheric pressure ionsource which uses nebulizer gas (which is not limited to the APCI ionsource) has another problem, i.e. a considerable amount of baselinefluctuation occurring in the spectrum. FIG. 10 shows an example of thespectrum observed with a conventional LC-APCI-IMS device in which thebaseline fluctuation is present.

The present invention has been developed to solve the previouslydescribed problems. Its first objective is to provide an ion mobilityspectrometer capable of preventing, as thoroughly as possible, theadhesion of the droplets to the needle electrode for corona discharge soas to maintain a stable corona discharge and thereby enable a stableionization even in the situation where the desolvation of the dropletsformed by the spraying process in the APCI ion source does not easilyprogress.

The second objective of the present invention is to provide an ionmobility spectrometer using an atmospheric pressure ion source, the ionmobility spectrometer being capable of reducing the amount of baselinefluctuation occurring in the spectrum.

Solution to Problem

The ion mobility spectrometer according to the present inventiondeveloped for achieving the aforementioned first objective is an ionmobility spectrometer in which a component in a sample is ionized undersubstantially atmospheric pressure and the produced ions are made toenter and drift through a drift region maintained at substantiallyatmospheric pressure so as to separate the ions according to their ionmobility, the ion mobility spectrometer including:

a) a sample spray section for spraying a liquid sample into anionization chamber maintained at substantially atmospheric pressure;

b) a needle electrode located within the ionization chamber on anopposite side to the drift region across a spray area into which theliquid sample is sprayed from the sample spray section, for inducingcorona discharge to generate a primary ion for generating asample-derived ion by reacting with a component in a sample sprayed fromthe sample spray section; and

c) a primary-ion-driving electric field creator for creating, betweenthe spray area and a primary ion generation area in which the primaryion is generated by corona discharge induced through the needleelectrode, an electric field for driving the primary ion present withinthe primary ion generation area toward the spray area.

In the ion mobility spectrometer according to the present invention, acompound in a liquid sample is ionized by an APCI method. Within theionization chamber, the spray area into which the liquid sample issprayed from the sample spray section, i.e. the area in which a largenumber of sample droplets are present, is spatially separated from theneedle electrode for inducing corona discharge for generating a primaryion. Therefore, even in the situation where the solvent cannot easilyvaporize from the sample droplets formed by the spraying process fromthe sample spray section, the sample droplets will not adhere to the tipof the needle electrode. This enables the corona discharge to bemaintained in a stable state. If the sample droplets, which are adielectric material, were adhered to the tip of the needle electrode,the electric field at the needle tip would be weaker and the coronadischarge would be unstable. Preventing an easy adhesion of the dropletsenables the corona discharge to be maintained in a stable state.Consequently, the primary ion for ionizing the sample component will begenerated in a stable manner.

The primary ion generated within the primary ion generation area aroundthe tip of the needle electrode by corona discharge is transferred tothe spray area under the effect of the electric field created by theprimary-ion-driving electric field creator. Therefore, despite thespatial separation between the tip of the needle electrode and the sprayarea, a sufficient amount of primary ions for causing a reaction withthe gas molecules of the target component generated from the dropletscan be supplied to the spray area. Accordingly, a high level ofionization efficiency can be achieved.

As one specific mode of the ion mobility spectrometer according to thepresent invention, the primary-ion-driving electric field creator mayinclude: a grid electrode facing the needle electrode, the gridelectrode arranged so as to partition the space between the primary iongeneration area and the spray area; and a voltage supplier for applyinga predetermined DC voltage to the grid electrode.

In this configuration, the primary ions generated within the primary iongeneration area by corona discharge move through the openings of thegrid electrode into the spray area. Although some ions may come incontact with the grid elements (electrode elements) of the gridelectrode and disappear, most of the primary ions can be efficientlyintroduced into the spray area, since the grid surface has asubstantially uniform potential distribution, which reduces thedisturbance of the equipotential surfaces in the electric field createdwithin the space between the grid surface and the tip of the needleelectrode. Additionally, the electrode included in theprimary-ion-driving electric field creator doubles as thecounter-electrode for the needle electrode to induce corona discharge.Using the flat grid electrode as this counter-electrode allows thedistance between the tip of the needle electrode and thecounter-electrode to be shortened. This enables a stable coronadischarge to easily occur even at a lower voltage.

As another mode of the ion mobility spectrometer according to thepresent invention, the primary-ion-driving electric field creator mayinclude: a ring electrode having a ring portion arranged at a positionwhere the opening plane of the inner opening of the ring portionpartitions the space between the primary ion generation area and thespray area; and a voltage supplier for applying a predetermined DCvoltage to the ring electrode. In this configuration, the primary ionsgenerated in the primary ion generation area by corona discharge movethrough the large opening of the ring electrode into the spray area.

In order for the sample-component-derived ions generated within a spacenear the spray area to be efficiently introduced into the drift region,the ion mobility spectrometer according to the present invention mayfurther include a target-ion-driving electric field creator forcreating, within a space between the spray area and an entrance of thedrift region, an electric field for driving a sample-component-derivedion toward the entrance of the drift region.

For example, the target-ion-driving electric field creator may include aplurality of ring electrodes and a voltage supplier for applying apredetermined DC voltage to each of the ring electrodes. As anotherexample, the target-ion-driving electric field creator may include atubular electrical resistor and a voltage supplier for applying apredetermined DC voltage to each of the two ends of the electricalresistor.

In any of these configurations, an electric field in which ions smoothlymove from the spray area toward the entrance of the drift region iscreated. In an ion mobility spectrometer, a stream of neutral gas (whichis normally an inert gas) is normally passed through the drift region inthe opposite direction to the travelling direction of the ions. Evenwhen such a gas stream is overflowing beyond the entrance of the driftregion toward the spray area, the sample-component-derived ionsgenerated within the spray area can be conveyed to the entrance of thedrift region, against the gas stream, by the effect of the electricfield. In this manner, the sample-component-derived ions generated in aspace near the spray area can be efficiently introduced into the driftregion and used for an analysis.

In the ion mobility spectrometer according to the present invention, itis preferable to maximally promote the vaporization of the solvent fromthe micro-droplets formed by the spraying process into the ionizationchamber. Within the drift region, a stream of heated neutral gas issupplied from the rear to the front end of the drift region to promotethe desolvation. In addition to this, the ion mobility spectrometer mayinclude: a heater for heating the ionization chamber; and a gasintroduction section for introducing a heated gas into a space betweenthe ionization chamber and the drift region, and a stream of the heatedgas introduced by the gas introduction section is created from theentrance of the drift region toward the spray area.

In this configuration, the micro-droplets are exposed to the dry heatedgas within the ionization chamber. Therefore, as compared to the casewhere the droplets are present in a simple high-temperature atmosphere,the vaporization of the solvent is further promoted, whereby thegeneration of the target ion originating from the sample is alsopromoted.

In some cases, the process of spraying the liquid sample into theionization chamber can be satisfactorily performed by simply sprayingthe liquid sample. However, in most cases, nebulizer gas is used toassist the spraying process. Accordingly, commonly used sample spraysections include a nebulizer gas tube for ejecting nebulizer gas. Withthe help of this nebulizer gas ejected from the nebulizer gas tube, theliquid sample is broken into fine droplets and sprayed into theionization chamber. As a result of various studies, the presentinventors have discovered that a vibration due to the nebulizer gasconstitutes a major cause of the baseline fluctuation which occurs in aspectrum obtained with an ion mobility spectrometer employing an APCI,ESI or similar type of ion source.

More specifically, if a faint vibration which accompanies the ejectionof the nebulizer gas is transmitted to a drift tube within which thedrift region is formed, a vibration current occurs in the detectorlocated at the rear end of the drift tube. There are also other elementswhich simultaneously undergo the vibration, such as a grid electrodewhich is normally provided in front of the detector in order to reducean image current induced in the detector by the ions flying into thedetector while being drifted, a preamplifier which amplifies the signalsobtained with the detector, and a signal cable which connects thedetector and the preamplifier. If an electric capacitance (electrostaticcapacitance) changes due to such a vibration, an electric currentcorresponding to that change occurs, which will be detected as abaseline fluctuation in the spectrum. Accordingly, in order to reducethe baseline fluctuation attributable to this factor, it is effective todirectly prevent the occurrence of the vibration due to the ejection ofthe nebulizer gas or check the transmission of this vibration to thedrift tube.

Accordingly, in a more preferable mode of the ion mobility spectrometeraccording to the present invention, an ion chamber forming theionization chamber and a drift tube having the drift region formedinside are provided as separate bodies, and the ion chamber and thedrift tube are fixed individually and independently. The ion chamber andthe drift tube may be constructed in a simply contactless form. It isalso possible to connect the ion chamber and the drift tube via anelastic member having the vibration-dampening effect.

In this configuration, even when the amount of nebulizer gas in the APCIion source is increased and the vibration due to the ejection of the gasis consequently increased, the baseline fluctuation due to the vibrationwill not occur since the vibration is prevented from reaching the drifttube. That is to say, the aforementioned second objective can beachieved.

Nebulizer gas is commonly used, not only in the APCI ion source but alsoin other types of atmospheric pressure ion sources, such as the ESI ionsource. In practice, the problem of the baseline fluctuation whichoccurs in a spectrum has also been recognized in other ion mobilityspectrometers that employ atmospheric pressure ion sources differentfrom the APCI ion source. Therefore, the aforementioned preferable modeis also useful in an ion mobility spectrometer which employs a differenttype of atmospheric pressure ion source.

Accordingly, an ion mobility spectrometer according to the second aspectof the present invention developed for achieving the aforementionedsecond objective is an ion mobility spectrometer in which a liquidsample to be analyzed is sprayed into an ionization chamber maintainedat substantially atmospheric pressure, a component in the sample isionized within the ionization chamber, and the produced ions aretransferred to a drift region maintained at substantially atmosphericpressure as well as made to drift through the drift region so as toseparate the ions according to their ion mobility, the ion mobilityspectrometer characterized in that:

an ion chamber forming the ionization chamber and a drift tube havingthe drift region formed inside are provided as separate bodies, and theion chamber and the drift tube are fixed individually and independently.

According to the second aspect of the ion mobility spectrometer, whichis an ion mobility spectrometer employing an atmospheric pressure ionsource configured to spray a liquid sample into an ionization chamberusing nebulizer gas, the vibration due to the ejection of the nebulizergas is prevented from reaching the drift tube, so that the baselinefluctuation in the spectrum due to the vibration will not occur.

Advantageous Effects of the Invention

The ion mobility spectrometer according to the present invention canproduce target ions originating from a sample by effectively using theprimary ions generated by corona discharge while preventing sampledroplets with insufficient vaporization of the solvent from adhering tothe needle electrode for corona discharge. As a result, no spike noisedue to an extinction of the corona discharge or due to a sudden surge ofthe discharge occurs in the spectrum, so that a high-quality spectrumreflecting the ions produced in a stable manner can be obtained.

In the ion mobility spectrometer according to the second aspect of thepresent invention, even when the supply of the nebulizer gas isincreased, the baseline fluctuation in the spectrum is effectivelysuppressed, so that a highly accurate spectrum can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic configuration diagram of an LC-APCI-IMS deviceaccording to the first embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of the APCI ion source andsurrounding elements in FIG. 1.

FIG. 3 is a schematic configuration diagram of an LC-APCI-IMS deviceaccording to the second embodiment of the present invention.

FIG. 4 is a schematic configuration diagram of an LC-APCI-IMS deviceaccording to the third embodiment of the present invention.

FIG. 5 is a schematic configuration diagram of an LC-APCI-IMS deviceaccording to the fourth embodiment of the present invention.

FIG. 6 is a schematic configuration diagram of an LC-APCI-IMS deviceaccording to the fifth embodiment of the present invention.

FIG. 7 is a schematic configuration diagram of an LC-ESI-IMS deviceaccording to the sixth embodiment of the present invention.

FIG. 8 is a schematic configuration diagram of an LC-APPI-IMS deviceaccording to the seventh embodiment of the present invention.

FIG. 9 is a graph showing one example of the chromatogram observed witha conventional LC-APCI-IMS device in which spike noises are present.

FIG. 10 is a graph showing an example of the spectrum (IMS spectrum)observed with a conventional LC-APCI-IMS device in which the baselinefluctuation is present.

DESCRIPTION OF EMBODIMENTS

Several embodiments of the ion mobility spectrometer according to thepresent invention will be described with reference to the attacheddrawings.

First Embodiment

FIG. 1 is a schematic configuration diagram of a liquid chromatographatmospheric pressure chemical ionization source ion mobilityspectrometer (LC-APCI-IMS device) according to the first embodiment ofthe present invention. FIG. 2 is a schematic configuration diagram ofthe APCI ion source and surrounding elements in FIG. 1.

This LC-APCI-IMS device is roughly divided into an LC unit 2 andAPCI-IMS unit 1. Though not shown, the LC unit 2 is provided with amobile phase supplier including a liquid-sending pump, an injector forintroducing a sample into a mobile phase, a column for separating thecomponents in a sample, and other elements, whereby a plurality ofcomponents contained in a sample are temporally separated. A liquidsample containing the separated components is continuously supplied tothe APCI-IMS unit 1.

The APCI-IMS unit 1 is provided with an ion chamber 10 with the innerspace forming an ionization chamber for ionizing the components in asample, and a drift tube 11 with the inner space forming the driftregion for separating ions by using their ion mobility. In the presentembodiment, the ion chamber 10 and drift tube 11 are combined into asingle body having a substantially cylindrical shape with a uniformdiameter. The ion chamber 10 and drift tube 11 are circumferentiallyprovided with heating blocks 12 and 13 for respectively heating them.

In the circumferential wall of the ion chamber 10, an APCI spray nozzle3 is attached in such a manner that the spraying direction of the liquidsample is substantially orthogonal to the central axis of the ionchamber 10. The central axis of the ion chamber 10 and that of the drifttube 11 coincide with each other. These axes are hereinafter simplyreferred to as the “central axis S”.

The spray nozzle 3, which is supplied with a liquid sample from the LCunit 2, serves to make this sample carried by a stream of nebulizer gas(which is normally an inert gas, such as nitrogen or helium) and sprayedinto the ion chamber 10 through a drying tube heated to hightemperatures (300-500° C.). Within the ion chamber 10, a needleelectrode 14 for inducing corona discharge is provided at the end whichis opposite to the end leading to the drift tube 11, and a first gridelectrode 15 having a large number of openings is stretched across thespace between this needle electrode 14 and the spray nozzle 3. Betweenthis first grid electrode 15 and the drift tube 11, a plurality of ringelectrodes 16 are arranged at predetermined intervals along theextending direction of the central axis S.

A plurality of ring electrodes 17 are similarly arranged within thedrift tube 11 at predetermined intervals along the extending directionof the central axis S, forming a continuation of the ring electrodes 16in the ion chamber 10. It should be noted that a shutter gate 18 (whichis a grid electrode) is provided in place of the ring electrode 17 at apredetermined position in the middle of the drift tube 11. Within thedrift tube 11, an ion detector 22 is placed at the end which is oppositeto the end leading to the ion chamber 10, along with a second gridelectrode 19 stretched across the space between this ion detector 22 andthe rearmost ring electrode 17. A gas introduction tube 20 is connectedto the circumferential wall of the drift tube 11 at a point near the iondetector 22. Through this gas introduction tube 20, neutral gas (e.g.nitrogen gas) is supplied into the drift tube 11 at a constant flowrate. As indicated by the thick chained line in FIG. 1, the neutral gassupplied into the drift tube 11 flows in the direction from the iondetector 22 to the needle electrode 14, to be discharged through theexhaust port 21 provided at the end of the ion chamber 10. The neutralgas passed through the drift tube 11 is normally heated to approximatelythe same temperature as the drift tube 11 (around 200° C.) before beingintroduced into the tube.

The first grid electrode 15, plurality of ring electrodes 16 and 17 aswell as second grid electrode 19 are individually connected to avoltage-dividing circuit 23 formed by an array of resistors. Thevoltage-dividing circuit 23 generates different levels of DC voltages bydividing a predetermined amount of DC voltage generated by a secondvoltage source 25, and those DC voltages are respectively applied to theaforementioned electrodes. Additionally, a high voltage of approximatelya few kV for corona discharge is applied from a first voltage source 24to the needle electrode 14, while a control voltage for controlling thepassage and blockage of ions is applied from a shutter gate controller26 to the shutter gate 18. The first voltage source 24, second voltagesource 25 and shutter gate controller 26 are controlled by a controlunit (not shown).

In the LC-APCI-IMS device of the present embodiment, the region labeled“A” within the ion chamber 10 in FIG. 1 is the ionization region, theone labeled “B” within the drift tube 11 is the desolvation region, andthe one labeled “C” within the same drift tube 11 is the drift region.In other words, a desolvation region B for promoting the solventvaporization for the sample-derived target ions (the ions contained inmicro-droplets) from which the solvent has been insufficiently vaporizedis provided between the ionization region A for ionizing the targetcomponents and the drift region C for separating and detecting the ions.

An operation for analyzing an ion originating from a target component ina sample in the LC-APCI-IMS device of the present embodiment ishereinafter described.

When a liquid sample containing the compounds separated by the LC unit 2reaches the APCI spray nozzle 3, the liquid sample is broken intomicro-droplets and sprayed into the ionization chamber with the help ofthe nebulizer gas. Since the ion chamber 10 is heated to appropriatetemperatures (normally, 150-300° C.) by the heating block 12, thesolvent contained in the micro-droplets is vaporized and the targetcomponents in the sample turn into gas molecules. Meanwhile, the highvoltage applied from the first voltage source 24 to the needle electrode14 creates an electric field concentrated at the tip of the thin needleelectrode 14. Since the distance between the tip of the needle electrode14 and the first grid electrode 15 is as short as a few to tenmillimeters along the central axis S, a corona discharge occurs due tothe non-uniform electric field formed between the tip of the needleelectrode 14 and the first grid electrode 15. This corona dischargeionizes the air around the tip of the needle electrode 14, neutral gascoming from the drift tube 11 and other substances, whereby primary ionsare generated.

As shown in FIG. 2, the primary ions are mostly generated within aprimary ion generation area 30 around the tip of the needle electrode14. The primary ion generation area 30 is separated from a spray area 31in which a large number of droplets sprayed from the APCI spray nozzle 3are present. Within the space between these two areas 30 and 31, anelectric field is created due to the DC voltages applied to the firstgrid electrode 15 and the ring electrodes 16. This electric field has apotential gradient which drives the primary ions along the central axisS toward the spray area 31. Due to the effect of this electric field,the primary ions existing in the primary ion generation area 30 passthrough the openings of the first grid electrode 15 and move toward thespray area 31. Upon reaching a space near the spray area 31, the primaryions react with the sample components gasified from or contained in thedroplets, whereby ions originating from those components are produced.

In this manner, despite the spatial separation between the primary iongeneration area 30 and the spray area 31, sample-component-derived ionscan be efficiently produced in a space near the spray area 31. Due tothe sufficient separation between the needle electrode 14 and the sprayarea 31 as well as the presence of the grid electrode 15 between them,the sample droplets cannot reach an area near the tip of the needleelectrode 14. In this manner, the adhesion of the sample droplets to theneedle electrode 14 is prevented, so that the corona discharge can beinduced in a stable state by the previously set application voltage.

The polarity of the sample-component-derived target ions to beeventually generated depends on that of the primary ions, while thepolarity of the primary ions is determined by that of the voltageapplied to the needle electrode 14. Accordingly, the polarity of thevoltage applied from the first voltage source 24 to the needle electrode14 should be changed according to the polarity of the target ion to beanalyzed. The polarity of the voltage generated by the second voltagesource 25 should also be changed accordingly.

The target ions generated in a space near the spray area 31 move towardthe shutter gate 18 under the effect of the electric field created bythe voltages applied to the ring electrodes 16 and 17. The ionsgenerated in the spray area 31 include not only the gas-phase targetions but also target ions which exist inside the droplets from which thesolvent has incompletely vaporized. The latter ions effectively act aselectrically charged droplets and move toward the shutter gate 18 alongwith the gas-phase target ions under the effect of the electric field.The drift tube 11 is heated to appropriate temperatures (normally,150-250° C.) by the heating block 13. The high-temperature neutral gasintroduced from the gas introduction tube 20 and flowing through thedrift region C passes through the desolvation region B between the spacenear the entrance end of the drift tube 11 and the shutter gate 18.Therefore, when passing through this desolvation region B, the chargeddroplets are exposed to the high-temperature neutral gas, whereby thevaporization of the solvent is further promoted and the target ions inthe droplets also turn into the gas phase.

Depending to the voltage applied from the shutter-gate controller 26,the shutter gate 18 periodically repeats the “open” state for allowingthe passage of the ions and the “closed” state for blocking the passageof the ions. The period of time during which the shutter gate 18 is inthe open state is sufficiently shorter than the drift time required fortarget ions to reach the detector 22 after passing through the shuttergate 18. Accordingly, the timing at which the shutter gate 18 is in theopen state is used as the starting point to measure the drift time.

A typical example of the shutter gate 18 is the so-called “BN(Bradbury-Nielsen) gate”. It has the line-and-space structure havingthin metallic wires with a thickness of approximately 100 μm stretchedat intervals of several hundred μm. When all neighboring metallic wiresare at the same potential, the shutter gate 18 is in the open state.When voltages which create a potential difference of approximately 100 Vbetween the neighboring metallic wires are applied, the shutter gate 18is in the closed state.

If insufficiently desolvated droplets were allowed to reach the shuttergate 18 and adhere to its metallic wires, unwanted electric dischargemight occur between the neighboring metallic wires, causing damage tothose metallic wires. The present ion mobility spectrometer preventssuch a situation by performing sufficient desolvation within thedesolvation region B before the shutter gate 18, as described earlier,so as to prevent the droplets from reaching the shutter gate 18.

The electric field created within the desolvation region B has apotential gradient which forms a downward slope for the target ions tomove toward the shutter gate 18. Therefore, when the shutter gate 18 isclosed, the target ions which have reached the shutter gate 18 stay atthe gate or become diffused. Then, when the shutter gate 18 is openedfor a short period of time, the staying target ions simultaneously passthrough the shutter gate 18 and enter the drift region C. While driftingthrough the drift region C, those ions are separated according to theirion mobility and reach the detector 22 through the second grid electrode19. The detector 22 produces detection signals corresponding to theamount of arriving ions and sends those signals to an external system.The second grid electrode 19 serves to prevent an image current frombeing induced in the detector 22 due to the motion of the ions in thevicinity of the detector 22. Preventing the occurrence of the imagecurrent has the effect of improving the rising characteristic of thedetection signal which is generated when a target ion arrives at thedetector 22.

As described thus far, the ion mobility spectrometer of the firstembodiment can achieve a high level of ion generation efficiency whilemaintaining a stable generation of the corona discharge by preventingthe adhesion to the needle electrode 14 of the droplets formed by thespraying process from the APCI spray nozzle 3.

Second Embodiment

FIG. 3 is a schematic configuration diagram of an LC-APCI-IMS deviceaccording to the second embodiment of the present invention. The samecomponents as used in the LC-APCI-IMS device according to the firstembodiment shown in FIG. 1 are denoted by the same numerals.

In the LC-APCI-IMS device of the second embodiment, a ring electrode 150similar to the ring electrodes 16 is provided in place of the first gridelectrode 15 in the LC-APCI-IMS device of the first embodiment. The ringelectrode 150 can similarly form an almost flat equipotential surfacewithin the inner space of its ring portion. The ring electrode 150 alsofunctions as the counter-electrode for the needle electrode 14.

Third Embodiment

FIG. 4 is a schematic configuration diagram of an LC-APCI-IMS deviceaccording to the third embodiment of the present invention. The samecomponents as used in the LC-APCI-IMS device according to the firstembodiment shown in FIG. 1 are denoted by the same numerals.

In the LC-APCI-IMS device of the third embodiment, in place of the ringelectrodes 16 in the LC-APCI-IMS device of the first embodiment, anelectrical resistor 160 having a cylindrical shape is provided along theinner circumferential wall of the ion chamber 10, with two predeterminedvoltages applied from the voltage-dividing circuit 23 to the two ends ofthe electrical resistor 160, respectively. In order to eliminate thepotential difference in the circumferential direction, it is preferableto attach an annular electric conductor to each end of the electricalresistor 160 and apply the aforementioned voltages to those electricconductors. Within the inner space of the electrical resistor 160, anelectric field having a linear potential gradient along the central axisS is created. Due to the effect of this electric field, the primary ionsgenerated in a space near the needle electrode 14 are driven toward aspace near the spray area. Furthermore, the target ions generated in aspace near the spray area are driven toward the entrance end of thedrift tube 11.

Fourth Embodiment

FIG. 5 is a schematic configuration diagram of an LC-APCI-IMS deviceaccording to the fourth embodiment of the present invention. The samecomponents as used in the LC-APCI-IMS device according to the firstembodiment shown in FIG. 1 are denoted by the same numerals.

In the LC-APCI-IMS device of the fourth embodiment, in order to improvethe ion separation performance, the drift region C is elongated byplacing the shutter gate 18 at the entrance of the drift tube 11.However, for this arrangement, the idea of providing the desolvationregion within the drift tube 11 is abandoned. Therefore, it is necessaryto even further promote the desolvation within the limited space of theion chamber 10. To this end, dry gas is supplied into the ion chamber 10through a dry gas introduction tube 200 whose outlet end is locatedbetween the ion chamber 10 and the drift tube 11. Additionally, thetemperature of the heating block 12 surrounding the ion chamber 10 isset at a higher level than the temperature of the heating block 13surrounding the drift tube 11. By such a system, the vaporization of thesolvent from the droplets in the ion chamber 10 is further promoted, sothat the situation in which the droplets reach the shutter gate 18 canbe avoided despite the shortened distance between the spray area and theshutter gate 18.

Fifth Embodiment

FIG. 6 is a schematic configuration diagram of an LC-APCI-IMS deviceaccording to the fifth embodiment of the present invention. The samecomponents as used in the LC-APCI-IMS device according to the firstembodiment shown in FIG. 1 are denoted by the same numerals. Unlike theLC-APCI-IMS device of the first embodiment in which the ion chamber 10and the drift tube 11 are combined into a single body, the ion chamber10 and the drift tube 11 in the LC-APCI-IMS device of the fifthembodiment are constructed as separated bodies, which are individuallyand independently held by fixation members (not shown) within the casingof the device. Although the gap between the ion chamber 10 and the drifttube 11 is narrow, a vibration isolator 27 is provided in this gap toprevent external air and/or suspended matters from entering the ionchamber 10 through the gap. By such a construction, the vibration of theion chamber 10 is prevented from reaching the drift tube 11.

As noted earlier, in the case of spraying a liquid sample usingnebulizer gas in the APCI spray nozzle 3, a faint vibration occurs inthe ion chamber 10 due to the spraying action. If this vibration wereallowed to reach the drift tube 11, the vibration would constitute afactor of the baseline fluctuation in the spectrum. However, in theLC-APCI-IMS device of the present embodiment, even when a faintvibration occurs in the ion chamber 10, the baseline fluctuation due tosuch a vibration will not occur since the vibration cannot reach thedrift tube 11. Needless to say, if the entry of external air or othersubstances through the gap between the ion chamber 10 and the drift tube11 does not cause any problem, the vibration isolator 27 for closing thegap is unnecessary.

Such a problem is not limited to the APCI ion source; a similar problemcan occur in any atmospheric pressure ion source constructed to spray aliquid sample into an ionization chamber by using nebulizer gas, such asthe ESI ion source or APPI ion source. Accordingly, the structure inwhich the ion chamber 10 and the drift tube 11 are independently held asseparate bodies as shown in FIG. 6 can also be applied in an ionmobility spectrometer employing an atmospheric pressure ion sourcedifferent from the APCI ion source, as will be hereinafter described.

Sixth Embodiment

FIG. 7 is a schematic configuration diagram of one embodiment of anLC-ESI-IMS device obtained by changing the ion source in the LC-APCI-IMSdevice of the fifth embodiment to an ESI ion source. The same componentsas used in the LC-APCI-IMS device according to the fifth embodimentshown in FIG. 6 are denoted by the same numerals.

In the LC-ESI-IMS device of the sixth embodiment, an ESI spray nozzle 4is attached to the circumferential wall of the ion chamber 10 in placeof the APCI spray nozzle. A liquid sample which has reached the ESIspray nozzle 4 is transformed into electrically charged droplets bybeing sprayed through a non-uniform electric field created by the highvoltage applied from a fourth voltage source 28 to the spray nozzle 4.The charged droplets become even finer particles by coming in contactwith neutral gas (or similar gas). Concurrently, the solvent isvaporized by heat. Through such a process, gas-phase target ions aregenerated.

It should be noted that the spraying direction of the liquid sample fromthe ESI spray nozzle 4 is not orthogonal to the central axis S butobliquely directed toward the shutter gate 18 at a certain angle to thecentral axis S. This is intended to help the charged droplets and thetarget ions produced from those droplets travel toward the shutter gate18.

Similarly to the fifth embodiment, the ion chamber 10 and the drift tube11 in the sixth embodiment are constructed as separate bodies and heldindependently. Therefore, the faint vibration which occurs in the ionchamber 10 cannot reach the drift tube 11, so that the baselinefluctuation in the spectrum due to the vibration of the drift tube 11will not occur.

Seventh Embodiment

FIG. 8 is a schematic configuration diagram of one embodiment of anLC-APPI-IMS device obtained by changing the ion source in theLC-APCI-IMS device of the fifth embodiment to an APPI ion source. Thesame components as used in the LC-APCI-IMS device according to the fifthembodiment shown in FIG. 6 are denoted by the same numerals.

In the LC-APPI-IMS device of the seventh embodiment, an APPI lightsource 29 is provided within the ion chamber 10. The APPI light source29 is arranged so that the emitted light illuminates a space near thespray area in which a large number of droplets sprayed from the spraynozzle 3 are present. Sample components are ionized by the effect ofthis light.

Similarly to the fifth embodiment, the ion chamber 10 and the drift tube11 in the seventh embodiment are constructed as separate bodies and heldindependently. Therefore, the faint vibration which occurs in the ionchamber 10 cannot reach the drift tube 11, so that the baselinefluctuation in the spectrum due to the vibration of the drift tube 11will not occur.

It should be noted that any of the previous embodiments is a mereexample of the present invention, and any change, modification oraddition appropriately made within the spirit of the present inventionwill naturally fall within the scope of claims of the presentapplication.

REFERENCE SIGNS LIST

-   1 . . . APCI-IMS Unit-   2 . . . LC Unit-   3 . . . APCI Spray Nozzle-   4 . . . ESI Spray Nozzle-   10 . . . Ion Chamber-   11 . . . Drift Tube-   12, 13 . . . Hearting Block-   14 . . . Needle Electrode-   15 . . . First Grid Electrode-   16, 17, 150 . . . Ring Electrode-   18 . . . Shutter Gate-   19 . . . Second Grid Electrode-   20 . . . Gas Introduction Tube-   21 . . . Exhaust Port-   22 . . . Ion Detector-   23 . . . Voltage-Dividing Circuit-   24 . . . First Voltage Source-   25 . . . Second Voltage Source-   26 . . . Shutter Gate Controller-   27 . . . Vibration Isolator-   28 . . . Fourth Voltage Source-   160 . . . Electrical Resistor-   200 . . . Dry Gas Introduction Tube-   A . . . Ionization Region-   B . . . Desolvation Region-   C . . . Drift Region

The invention claimed is:
 1. An ion mobility spectrometer comprising: anion chamber forming an ionization region; a spray nozzle attached on acircumferential wall of the ion chamber, the spray nozzle beingconfigured to spray a liquid sample by using a nebulizer gas; a drifttube having a drift region formed inside; a detector located at a rearend of the drift tube; and a vibration isolator provided between the ionchamber and the drift tube so as to be in direct contact with an innersurface of the ion chamber and an outer surface of the drift tube, thevibration isolator being configured to prevent transmission of vibrationdue to ejection of the nebulizer gas, wherein: the liquid sample issprayed into the ionization region maintained at atmospheric pressurethrough the spray nozzle, the liquid sample is ionized within theionization region, and produced ions are transferred to the drift regionmaintained at atmospheric pressure made to drift through the driftregion so as to separate the produced ions according to their ionmobility; and detected by the detector to obtain an ion mobilityspectrum; the ion chamber and the drift tube are provided as separatebodies, and the ion chamber and the drift tube are fixed individuallyand independently; and a vibration current in the detector is suppressedby the vibration isolator so that a baseline fluctuation in the ionmobility spectrum is reduced.
 2. The ion mobility spectrometer accordingto claim 1, wherein the vibration isolator is formed by an elasticmember.